Presently, lithium-ion electrolyte layers are deposited from ceramic, insulating sputter targets using radio-frequency (RF) excitation (1 MHz-1 GHz) on the sputter target together with a RF tuner and a RF matching network. The main reason for this approach lies in the fact that the ceramic target composition or stoichiometry can be fabricated identical, or at least most similar, to the stochiometry of the lithium-ion electrolyte layer to be deposited. However, both the use of the ceramic insulating sputter target and the RF sputter method may be undesirable due to their associated high costs, limitations in deposition area and rate, and hardware sophistication and challenges.
The electrically insulating type target typically requires the use of RF excitation when sputtering. Otherwise, when not using RF excitation, such as direct current (DC), pulsed DC (e.g. 250 kHz), or alternate current with a lower frequency (AC; e.g. 100 kHz) excitation, the target surface charges up substantially and releases the built-up charges in form of an electric arc to the substrate, the chamber wall, and/or the dark space shield. Electric arcing could be either so severe that electronics in the power supply connected to the sputter target may not prevent this event from occurring, which detrimentally affects the film growth on the substrate, or the sputter process could be interrupted too frequently by the preventive electronics of the power supply that a sputter deposition may not take place. Unfortunately, the entire RF electronics around a sputter target are fairly expensive as well as require the vacuum deposition chamber to be RF compatible. Most large sputter deposition tools used in semi-conductor manufacturing are not RF compatible but are only direct current (DC) or pulse DC compatible. The design and build of new, large vacuum deposition chambers takes substantial amounts of time and money because these chambers are not anticipated to be built and sold in large numbers.
Another issue in RF sputtering is that, for deposition of oxide dielectric films, ceramic targets are typically formed of multiple smaller tiles due to limitations in fabricating large area ceramic sputter targets in thicknesses (e.g. ¼ inch) that are appropriate for use in sputter processes. Further, the reactors required for RF sputtering tend to be rather complicated. In particular, the engineering of low capacitance efficient RF power distribution to the sputter cathode is difficult in RF systems. Routing of low capacitance forward and return power into a vacuum vessel of the reaction chamber often exposes the power path in such a way that diffuse plasma discharge is allowed under some conditions of impedance tuning of the matching networks.
Typically, it has been difficult to fabricate large area, insulating, ceramic sputter targets because their constituent tile size, singly-tiled or multi-tiled, is limited by today's available ceramic processing methods (cold pressing plus subsequent sintering or, instead, hot pressing of appropriate starting powders) in light of the performance requirements for ceramic sputter target tiles when sputtered under the thermal stresses of high rate sputter deposition. However, owing to the inherent brittleness of ceramic tiles and their limited, practical thickness when to be used in magnetron sputter targets (typical thickness is about ½ to ¼ inch for practical usage purposes) wherein the magnetic field of the magnetron has to go well through the sputter target tile thickness, tile manufacturers face the challenging task of making the target tiles as large as possible while being limited in its thickness. Thus, target manufacturers have encountered a loosely defined limit for the area/thickness ratio for every target tile material above which the target tile fabrication yield becomes too low to be economically viable. For practical and well performing Li3PO4 sputter target tiles, the tile size limit today for ¼ inch thick targets is on the order of 10″ in diameter or 7″×7″ for non-disc shapes.
In addition to the difficult or even impossible fabrication of large area, insulating, ceramic sputter targets that prove to be mechanically sufficiently resilient in high-rate sputter depositions, issues such as local charging/arcing, cross talk with other areas, and severe and variable impedance mismatch between the power supply and deposition environment conspire to limit the nominal practical sputter target area to be below about 1000 cm2 for well established ceramic sputter materials such as Al2O3 and about 500 cm2 for less established ceramic materials, such as Li3PO4.
In light of the issues involved in ceramic tile fabrication and the associated use of RF target excitation, it is desirable to switch from ceramic to metallic target tiles, if possible with respect to the necessary reactive sputter deposition in the case of metallic tiles, because metallic plates of about ¼ inch in thickness can more easily be fabricated in large areas. Another inherent benefit of using metallic targets over ceramic targets is based on the fact that the far more ductile metallic targets can be sputtered at much higher deposition powers and deposition rates, which creates a stressful temperature gradient inside a sputter target tile with which metallic target tiles can cope much more readily compared to brittle ceramic tiles.
The economics of mass-producing lithium-ion thin film electrochemical storage and conversion devices strongly depend on the capital expenditures for a given production throughput, which in turn is affected by the deposition rate, deposition area, deposition yield, and equipment up-time. In this regard, the issues of having to use relatively small, brittle ceramic sputter targets or target tiles in conjunction with RF sputter target excitation represents a significant economic barrier in scaling the production processes of lithium-ion thin film electrochemical storage and conversion devices to industrial levels.
To avoid the cost and tool issues involved in RF sputtering one may seek to sputter deposit said electrolyte or dielectric layers from electrically conducting sputter targets using DC or pulsed DC target excitation power. In that case, the charge-up and electric arcing issues are fewer and typically manageable. Both of these DC sputter methods are less expensive than RF and simpler to implement in vacuum deposition systems. However, in order to attain an electrically insulating but ionically (here: lithium-ion) conducting electrolyte or dielectric layer from an electrically conducting sputter target, one has to sputter deposit the target material in a reactive atmosphere to achieve the electrically insulating film composition of the correct stoichiometry. In some cases one may opt to attain the correct film stoichiometry via co-sputtering from a suitable, second sputter target, which, if electrically insulating, would require RF power excitation whereas if electrically conducting, could be sputtered by DC or pulsed DC excitation as well.
There is not a substantial track record in the field of creating sputter deposition targets that are amenable to DC or pulse DC deposition while producing insulating/dielectric, lithium containing films with electrolytic properties when deposited. Nor is there a significant amount of published work on the subject of using alternative sputter target compositions or configurations specifically to allow for DC deposition of sputtered materials that turn into electrolytes by becoming electrically insulating and ionically conducting when deposited as well. This scarcity of published information or patents extends to not only lithium-ion but all electrolytes. Most of the published work in this area focuses on how to increase the target area or how to improve the hardware. Whereas, the motivation of the present invention is to fabricate a conductive target composition that allows the use of DC or pulsed DC target sputter power to accomplish a lithium-ion electrolyte thin film cheaper and faster.
U.S. Patent application No. 2006/0054496 discloses oxide and oxynitride films being presented and deposited by a DC sputter method from a metallic target material. This disclosure focuses, however, on the sputter hardware and does not, for example, address the problem of modifying the composition/structure of the target to facilitate DC powered deposition of sputter materials whose physical vapor deposited films turn into thin-film electrolytes.
U.S. Pat. No. 5,753,385 (the “'385 patent”) uses metallic sputter targets of zirconium and yttrium to form oxides that are used as high-temperature, oxygen-ion conducting membranes in solid oxide fuel cells. Although these membranes have electrolytic properties, they are only high-temperature electrolytic properties and are exclusively relative to oxygen ions. The present invention, in contrast, focuses, for example, on ambient-temperature lithium-ion electrolytes. The underlying chemistry and physical parameters to form oxygen-ion electrolytes and lithium-ion electrolytes are very different. For example, a high-temperature oxygen-ion electrolyte needs to be crystalline while a lithium-ion electrolyte of the present invention only needs to be glassy or amorphous. In fact, if the lithium-ion electrolyte of the present invention is or becomes crystalline during its fabrication or thereafter at any time during its lifetime, it will severely limit the associated lithium thin-film electrochemical storage and conversion device to the use of only non-metallic lithium-ion anodes because a metallic lithium anode may short-circuit a lithium thin-film electrochemical storage and conversion device by creating an electrochemical short-circuit pathway from the anode to the cathode via grain boundary diffusion inside the crystalline electrolyte. If no (glassy or amorphous lithium-ion electrolyte) or virtually no (nano-crystalline lithium-ion electrolyte) grain boundaries exist, the inadvertent and undesirable formation of an electrochemical short-circuit pathway may not occur. The reason why the '385 patent can afford the use of a (high-temperature oxygen-ion) crystalline electrolyte lies mostly in the fact that the fuel cell device does not possess a creeping, easily diffusing, short-circuit creating, metallic lithium anode. Due to the vast difference of the motivation as well as benefits between the current invention and the '385 patent, one of ordinary skill in the art would not come up, for example, with a way to fabricate a lithium-ion thin-film electrolyte by non-RF sputter deposition techniques based on the '385 patent.
U.S. Pat. No. 7,179,350 B2 discloses a hybrid approach optimized for reactive sputtering, whereby the magnetron head is driven by both DC and RF controllers at the same time. Its focus, however, is on sputter hardware modification and not on target composition to achieve a lithium electrolyte film.
While battery and fuel cell thin film electrolytes have been radio-frequency (RF) sputter deposited for more than one decade, the instrumentation used for these growths has been relatively small: research and development or pilot line systems only. As this technology is scaled to the necessary (profitable) full industrial level, there is a need for more economical equipment and less costly consumables. Accordingly, the ability to sputter deposit electrolyte and dielectric layers over very large areas more quickly, and with less expense by using larger metallic targets and less expensive DC type sputter target excitation is very appealing, and the proper solution should result in significant savings of both time and money during high volume production.